I. Field of the Invention
This invention relates to a very high-speed, time-division-multiplexed, serial, data bus, which is capable of supporting very high data rates among a multiplicity of users over a large geographical area, and more specifically, to a system which is capable of supporting communication with up to 239 nodes distributed over a radius distance of 30 kilometers from the head end with an aggregate data rate of 500 megabits per second. Prior art approaches are not capable of this combined performance. In a more general sense, the system hardware of the present invention can transmit over a distance where the propagation time does not exceed 17 seconds.
II. Discussion of the Prior Art
Very high-speed, point-to-point data links are known in the prior art. It is also well understood in the art to have local area networks over a limited geographical distance operating at lower data rates, typically one to ten megabits per second. Such networks are comprised of a multiplicity of transmit/receive nodes which share a common transmission media. The combination of a very high data rate bus, operating over large distances, create a unique problem because of the time required for the propagation of data through the transmission media, even though such propagation may occur at a rate as high as one nanosecond per foot. Even with such high propagation rates, several hundred thousand bits of data may be suspended or latent in the transmission media at any instant in time. In prior art time division multiplexed data bus communication networks, where only one transmission source may be active, it is a common practice, when switching transmission source from a first node (node A) to a second node (node B) to allow time for the first transmission from node A to propagate to the end of the transmission media before transmission is allowed to begin on node B. The time for this to occur is an unuseable amount of time on the data bus, and thus results in a degradation of the net bandwidth or throughput of the bus. As the product of the total delay and the data rate increases, this becomes a more significant and, ultimately, a limiting factor on the performance of the bus.
Another factor limiting the performance of a digital bus is the method of bus control employed. Many techniques are well understood in the prior art and fall into one of two general categories. Either the bus assignments are (1) made on a fixed, invarient allocation, in which case, the versatility of the system suffers in that the bus cannot adapt to changing data loads between various nodes or (2) a form of dynamic bus control is implemented, which provides versatility, but at the penalty of introducing a dead-time period for bus arbitration.
One known method of bus control is referred to as Collision Sense Multiple Access (CSMA). This approach allows spontaneous contention for the bus by any of the plurality of nodes. In the event of a simultaneous attempt by more than one node to transmit on the bus, this collision is sensed and each node must wait a specified, but different, amount of time before attempting to transmit again. Eventually, with enough tries, any node can gain control of the bus. However, the access within a specified period of time is not guaranteed. As the density of message traffic increases on this bus, the probability of a collision increases and each collision is a loss of throughput on the bus. The loss of throughput increases as a function of both the distance and the data rate making this unacceptable for the class of applications contemplated by the present invention.
Another prior art method is referred to as the "token passing" method. In this case, a specific stream of data bits, called the "token", is transmitted sequentially around from node to node connected in a ring-fashion on the bus. Only the node currently holding the token is privileged to transmit. Collisions are avoided and the maximum throughput of the bus can be maintained. The penalty, however, is that the latency of the bus increases, due to the time it takes for the token to propagate around the ring. In most typical telecommunications applications, an increase in bandwidth cannot be effectively utilized without a corresponding decrease in latency. Since the latency in the token ring scheme is proportional to distance, this method is unusable for the very high speed geographically dispersed communications bus.
Another prior art method which is compatible with fiber optic bus technology is referred as the token bus. In this instance, the token does not pass sequentially from node to node but is granted to a given node through a token arbitration process. While this can reduce the problem of latency associated with the token ring, it suffers from loss of throughput as does the CSMA bus control method due to the token arbitration process and the time required to purge the network of suspended data.
Still another known digital data communication method is referred to as frequency division multiplexing. Channels are assigned different frequency bands very much like cable television. Specific nodes are then paired to transmit and receive on a specified channel. This method can achieve very high aggregate bandwidth/data rates, but at the expense of connectability. More particularly, the assignment of transmitting nodes to receiving nodes is made by the choice of frequency tuning of the transmitters and receivers at each node and once set, they are not easily changed. This is a distinct disadvantage over the time division multiplexing method in which any node can communicate with any other node connected to the bus.